Fluid Heater

ABSTRACT

A fluid heater ( 100 ) comprising a storage reservoir ( 1 ) for storing heating fluid, a burner ( 5 ) for heating the heating fluid and a first heat exchanger ( 11 ) located outside said storage reservoir ( 1 ). The heater ( 100 ) also comprises a first fluid circuit ( 200 ) connecting the storage reservoir ( 1 ) with the first heat exchanger ( 11 ) for carrying heating fluid from the storage reservoir ( 1 ), through the first heat exchanger ( 11 ) and back to the storage reservoir ( 1 ), and a second fluid circuit ( 300 ) connecting a first inlet ( 19 ) for mains fluid to a first outlet ( 27 ) for heated mains fluid via the first heat exchanger ( 11 ). The first heat exchanger ( 11 ) is adapted to exchange heat between said heating fluid and said mains fluid.

TECHNICAL FIELD

The present invention relates to a fluid heater and in particular to a water or oil heater.

The invention has been developed particularly for use as an instantaneous water heater and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular use and may also be used to heat other fluids, such as oil.

BACKGROUND OF THE INVENTION

Conventional gas and oil fueled water heaters can be classified into two types: storage; and instantaneous (continuous flow).

Conventional gas and oil fueled storage water heaters heat the water stored in an insulated tank. The stored hot water is drawn upon as required. When the hot water is used, it is replaced with cold water, which is re-heated and again stored in the tank.

There are several known deficiencies in the performance of gas and oil fueled storage water heaters. Because gas and oil fueled storage water heaters can only store a set volume of water they are limited in their ability to always supply hot water on demand. If the stored water runs out, the user has to wait until the heater re-heats the cold water to the temperature required by the user. The delay in re-heating the stored water is prolonged by the gas or oil input in the combustion chamber which is generally very low to ensure a high combustion efficiency (thermal efficiency) performance.

Conventional “high efficiency” gas and oil fueled storage water heaters generally have a low gas or oil input of between 25 and 50 megajoules, which provides a combustion (thermal) efficiency between 78 and 82%. If a conventional storage unit increases its gas or oil input to attempt a faster re-heating of the stored water its efficiency is lowered to an extent that it cannot comply with minimum efficiency requirements which have been set by governing authorities throughout the world.

Testing methods to rate energy efficiency of gas and oil fueled storage water heaters have continually evolved to ensure an accurate representation of total efficiency. The minimum performance/efficiency requirement which has been set by governing bodies has over the decades raised the minimum performance in relation to combustion (thermal) efficiency and insulation (maintenance loss) from the storage tank.

Conventional gas and oil fueled storage water heaters initially used the bottom of the storage tank and a central flue (through the middle of the tank) to heat the water. Over the last few decades manufacturers have increased the surface heat exchange area of the flue combustion products by utilising the exterior tank walls of the tank to further increase the s combustion (thermal) efficiency. The potential to increase the efficiency of storage tanks has virtually reached its limit due to the fact that manufacturers have increased the surface heat exchange area of the combustion products to virtually the entire exterior tank area.

Any large volume of stored water that is heated to a temperature above 55° C. and usually up to over 70° C. (to maximise water delivery) via a gas or oil burner (which heats the water by heat exchanging the high temperature combustion products against the tank wall or through the central flue) cannot maintain a combustion efficiency above 80 to 84% on an ongoing basis in domestic or commercial applications. Any combustion products which are driven or flow through a flue/heat exchanger mechanism which is part of a storage tank cannot drop the flue products temperature below the stored water temperature. The flue products must exit the unit at a higher temperature than the stored water in the tank.

The capacity to increase the combustion/thermal efficiency of a conventional gas or oil fueled storage water heater is limited by the heat/temperature retained in the stored water itself. It is impossible to lower the combustion products temperature below the temperature of the tank and consequently heat energy will always be lost to the air (via the flue). Conventional “high efficiency” gas/oil storage water heater's conversion of energy (fuel) to hot water is only 80 to 84%. The 20 to 16% lost is discharged into the atmosphere via the flue.

Conventional gas and oil fueled storage heaters waste approximately 6 litres of water per day via their safety pressure relief valve which ensures that the tank does not rupture during the re-heating cycle whereby the water expands up to approximately 3% inside the tank. The larger the volume of hot water used from a storage tank, the larger the water losses due to expansion upon re-heating. The expansion in the tank is caused by the mains water being trapped (at mains pressure) between the one way (non return) valve (on the mains water inlet line) and the outlet tap/taps whilst the burner is on and heating up the stored water in the tank to its thermostat off setting.

Instantaneous water heaters were first developed as an alternative to conventional storage water heaters. Instantaneous and continuous flow water heaters maintain a relatively high combustion/thermal efficiency. However, they also suffer several deficiencies in performance which result in energy and water wastage.

Conventional instantaneous water heaters are managed by a sophisticated thermal and flow control mechanism, which despite its complexity can not stabilise delivery under s certain draw off conditions which a storage system can achieve whilst it retains hot water in its reserve to continue delivery of hot water.

Instantaneous water heaters become cold in standby mode and have no hot water storage capacity. This feature ensures that there are no maintenance losses from the unit during its “off” period (i.e. after the heater has cooled down, after a heating period, and lost its residual heat to the atmosphere). Conversely storage tanks have a large tank surface area, which continually loses energy in standby mode despite the insulation around the tank.

The instantaneous unit's burner is activated by a water flow sensor which activates the gas or oil burner when the unit senses water flowing though the heater. Because the unit is cold when the water starts flowing through it, there is a significant delay whilst the burner is heats up the cold water heat exchanger which has cold (mains) water flowing through it. Consequently the first several litres of water which leave the unit are cold and the water (which is still flowing) slowly heats to the user's desired temperature. Whilst the user is waiting for the flowing water to heat up, the water output from the heater is wasted. This is not only a waste of water, but also of the gas or oil energy used in the heating up process.

After the mains water has been turned off (stopped flowing) the water heat exchanger inside the combustion chamber and the combustion chamber case/fixtures remains very hot. Because the mains water is no longer flowing through the unit the (instantaneous) heater has no method of capturing or storing the remaining heat which quickly dissipates into the atmosphere and is lost/wasted.

Instantaneous water heaters can waste up to 25 litres per day (or more) on a typical domestic user pattern because of the delay the unit suffers on start up as described above. The more times the unit is started from a cold start equates to more water and energy losses from the same cause.

Conventional instantaneous water heaters have a combustion/thermal efficiency of approximately 80% when the unit has stabilised the hot water delivery temperature. Instantaneous units may have a higher combustion/thermal efficiency for a short period just after the burner activates due to the water heat exchanger being cold however this higher efficiency performance is negated by the fact that the water flowing though the unit (during the short high efficiency period) is not hot enough to use by the user and is consequently wasted.

Some instantaneous units are fitted with a secondary (mains water) pre-heating heat exchanger, which can raise the efficiency to the around 90%. However, the use of a pre-heating heat exchanger has no significant effect on the water or energy wastage or the associated delays in supplying hot water previously described.

Conventional instantaneous water heaters are preset to not turn on/activate in circumstances where the water flow is below a 2 to 4 litres per minute flow rate. This design deficiency leads to circumstances whereby the user is required to turn the water flow up to over the 2 to 4 litre per minute activation (lighting) water flow setting, even though the requirement for the drawn off water may be significantly smaller. Consequently, more water and energy is wasted to achieve a small (hot) mains water draw off. This design deficiency can also be exacerbated in circumstances where the mains water supply pressure has been reduced and water saving devices have been fitted to the shower outlet. In these circumstances instantaneous water heaters have been known to not turn on at all or cut out and turn off during the shower, even though the user is attempting to run a 9 litre per minute shower. A typical storage system does not waste mains water in this fashion because the first volume of water that leaves the tank is hot, and the storage tank can supply hot water at any flow with no flow restriction being applied to it on the basis that it has enough storage capacity to continue supply at the draw off rate. If the stored water runs out the user has to wait a considerable period for the storage heater to re-heat the tank.

Conventional “high efficiency” gas and oil fueled storage and instantaneous/continuous flow water heaters have a combustion (thermal) efficiency of around 80%. As previously described, conventional gas or oil storage water heaters cannot maintain a combustion/thermal efficiency of around 90% and both types are limited in their potential to achieve a combustion/thermal efficiency of around 90% in general use.

In areas of poor domestic water quality, it is common for the water supply to contain impurities such as mud, silt and salt. These impurities can deposit themselves inside water heater components and/or tanks, which come into direct contact with the impurities. In instantaneous water heaters, the impurities can accumulate on the inner pipe walls of the combustion chamber (hardening) and over time can restrict the flow of water and ultimately cause the pipes to be blocked. This blocking of the pipes is commonly called scaling, and can cause water heaters to fail.

Conventional mains pressure storage water heaters can also fail when they are supplied with water which is corrupted with impurities, such as mud, silt or salt, which can collect at the bottom of the tank and sit/collect on the bottom tank dome, which is used as a heat exchange surface area by the gas combustion chamber. Over time the build up of sediment causes the heat exchange performance to be detrimentally effected and the efficiency of the unit continually drops as the build up of sediment increases. Ultimately the build up of sediment will prevent the mains water in the tank from scrubbing the bottom heat exchange dome, which prevents the dome (combustion chamber surface) from being cooled by the mains water. Over time the bottom tank dome will overheat, distort, and ultimately rupture the tank.

OBJECT OF THE INVENTION

It is the object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages.

SUMMARY OF THE INVENTION

Accordingly, in a first aspect, the present invention provides a fluid heater comprising:

-   -   a storage reservoir for storing heating fluid;     -   a burner for heating the heating fluid;     -   a first heat exchanger located outside said storage reservoir;     -   a first fluid circuit connecting the storage reservoir with the         first heat exchanger for carrying heating fluid from the storage         reservoir, through the first heat exchanger and back to the         storage reservoir; and     -   a second fluid circuit connecting a first inlet for mains fluid         to a first outlet for heated mains fluid via the first heat         exchanger,     -   wherein the first heat exchanger is adapted to exchange heat         between said heating fluid and said mains fluid.

The first heat exchanger preferably includes a first pipe-in-pipe heat exchanger. The first pipe-in-pipe heat exchanger preferably includes a first thermally conductive pipe for containing the heating fluid, said first thermally conductive pipe being located within a second thermally conductive pipe for containing the mains fluid. Alternatively, the first thermally conductive pipe is adapted to carry the mains fluid and the second thermally conductive pipe is adapted to carry the heating fluid. The first heat exchanger preferably includes a plurality of the first and second thermally conductive pipes, with each said first thermally conductive pipe being located within a respective said second thermally conductive pipe. The first thermally conductive pipes are preferably connected in parallel and the second thermally conductive pipes preferably loop backward and forward through the first heat exchanger. The second thermally conductive pipes preferably include a first group of pipes connected in parallel to a second group of pipes.

The first heat exchanger is preferably adapted to be heated by combustion products generated by the burner. The burner is preferably a gas burner and the first heat exchanger is preferably located above the gas burner, such that combustion products from the gas burner rise toward the first heat exchanger. The first heat exchanger preferably includes an array of fins for absorbing heat from the combustion products and transferring said heat to the mains fluid and/or heating fluid in said first heat exchanger.

A second heat exchanger, for exchanging heat between the heating fluid and the mains fluid, is preferably provided downstream of the first heat exchanger. The second heat exchanger is preferably located below the first heat exchanger, such that combustion products passing the second heat exchanger rise toward the first heat exchanger to heat the first heat exchanger. The second heat exchanger preferably includes a second pipe-in-pipe heat exchanger. The second pipe-in-pipe heat exchanger preferably includes a third thermally conductive pipe for containing the mains fluid, said third thermally conductive pipe being located within a fourth thermally conductive pipe for containing the heating fluid. Alternatively, the third thermally conductive pipe is adapted to carry the heating fluid and the fourth thermally conductive pipe is adapted to carry the mains fluid. The second heat exchanger preferably includes an array of fins for absorbing heat from the burner and transferring said heat to the mains fluid and/or heating fluid in said second heat exchanger. The second heat exchanger preferably includes a plurality of the third and fourth thermally conductive pipes, with each said third thermally conductive pipe being located within a respective said fourth thermally conductive pipe. The third thermally conductive pipes are preferably connected in parallel and the fourth thermally conductive pipes preferably loop backward and forward through the second heat exchanger. The fourth thermally conductive pipes are preferably connected in series. The second fluid circuit preferably includes a portion that bypasses the second heat exchanger.

A third heat exchanger, for exchanging heat between the heating fluid and the mains fluid, is preferably provided downstream of the first heat exchanger, and more preferably also downstream of the second heat exchanger. The third heat exchanger is preferably a pipe-in-pipe heat exchanger. More preferably, the third heat exchanger includes a fifth thermally conductive pipe for containing the mains fluid, said fifth thermally conductive pipe being located within a sixth thermally conductive pipe for containing the heating fluid. Alternatively, the fifth thermally conductive pipe is adapted to carry the heating fluid and the sixth thermally conductive pipe is adapted to carry the mains fluid.

The first fluid circuit is preferably defined in part by a first fluid conduit extending from the storage reservoir through the first heat exchanger, through the second heat exchanger, and back to the storage reservoir. The first fluid circuit is preferably defined in part by a third fluid conduit extending from the storage reservoir through the third heat exchanger, through the second heat exchanger and back to the storage reservoir. The third fluid conduit preferably intersects the first fluid conduit upstream of the second heat exchanger.

A first valve is preferably provided in the first fluid conduit for selectively preventing flow of the heating fluid through the first heat exchanger. The first valve is preferably located between the first heat exchanger and the intersection with the third fluid conduit. The first valve is preferably a solenoid valve. A second valve is preferably provided in the first fluid conduit for selectively preventing flow of the heating fluid from the storage reservoir via the first fluid conduit. The second valve is preferably located between the storage reservoir and the first heat exchanger. The second valve is preferably a solenoid valve.

An expansion valve is preferably provided between the second fluid circuit and the first fluid circuit to allow mains fluid to pass from the second fluid circuit into the first fluid circuit when pressure within the second fluid circuit increases to a predetermined level.

A first pump is preferably provided for pumping heating fluid through the first fluid circuit.

A second outlet is preferably provided in the first fluid circuit to allow for supply of the heating fluid to a fourth heat exchanger for exchanging heat between the heating fluid and fluid in an external hydronic heating circuit. The second outlet is preferably provided in a fourth fluid conduit that branches off from the first conduit downstream of the second heat exchanger. A second inlet is preferably provided in the first fluid circuit to allow for the return of heating fluid from the fourth heat exchanger. The second inlet preferably feeds into a fifth fluid conduit that feeds into the first heat exchanger. A third valve is preferably provided for selectively disconnecting flow of heating fluid through the fifth fluid conduit. The third valve is preferably a solenoid valve. A second pump is preferably provided in the fourth or fifth conduits for pumping heating fluid through the fourth and fifth fluid conduits and the fourth heat exchanger. The fourth heat exchanger is preferably a plate heat exchanger.

A valve is preferably provided for selectively controlling the supply of fuel to the burner.

A plurality of temperature and flow sensors are preferably provided for sensing the temperature and flow of fluids at various locations within the fluid heater. A controller is preferably responsive to the sensors for controlling flow of fluid through the fluid heater for safety and/or to allow the fluid heater to operate in various modes. A first temperature sensor is for sensing the temperature of the heating fluid adjacent its point of return to the storage reservoir. A second temperature sensor is provided for sensing the temperature of the heating fluid adjacent its point of return to the storage reservoir. A third temperature sensor is provided for sensing the temperature of the heating fluid downstream of the second heat exchanger. A first flow sensor is provided for sensing the flow of mains fluid adjacent the first inlet. The first flow sensor is preferably a thermal sensor, which senses flow based on the relative temperatures of the mains fluid and the heating fluid. The first flow sensor is preferably located between the mains fluid circuit and the heating circuit adjacent the first inlet. The controller is preferably also responsive to controls of the hydronic heating circuit.

The controller is preferably adapted to close the first valve in response to the first flow sensor sensing flow of mains fluid, to thereby prevent heating fluid from flowing through the first heat exchanger. The controller is preferably adapted to actuate a heating sequence of the burner in response to the first flow sensor sensing flow of mains fluid and the first temperature sensor indicating a temperature below a first predetermined value, preferably of approximately 82° C. The controller is preferably adapted to deactivate the burner in response to the first flow sensor sensing flow of mains fluid and the first temperature sensor indicating a temperature above a second predetermined value, preferably of around 85° C.

The controller is preferably adapted to open the first valve in response to the first flow sensor sensing a stoppage of mains fluid flow, to thereby allow heating fluid to flow through the first heat exchanger to scavenge residual heat in the first heat exchanger. The controller is preferably adapted to actuate the burner in response to the first flow sensor sensing a stoppage of mains fluid flow and the second temperature sensor indicating a temperature below a third predetermined value, preferably of around 65° C. The controller is preferably adapted to deactivate the burner in response to the first flow sensor sensing a stoppage of mains fluid flow and the second temperature sensor indicating a temperature above a fourth predetermined value, preferably of around 80° C.

The controller is preferably adapted to actuate the first pump and the second pump in response to the hydronic heating system being activated. The controller is preferably adapted to open the third valve in response to the hydronic heating system being activated. The controller is preferably adapted to close the second valve in response to the hydronic heating system being activated. The controller is preferably adapted to close the third valve and deactivate the second pump if the mains water temperature at the first outlet falls below a predetermined value.

An inlet to the first fluid circuit from the storage reservoir is preferably spaced apart from an outlet from the first fluid circuit into the storage reservoir. More preferably, the inlet and outlet are located at diametrically opposite sides of the storage reservoir.

The fluid heater preferably includes an insulated housing.

In a second aspect, the present invention provides a fluid heater comprising:

-   -   a storage reservoir for storing heating fluid;     -   a burner for heating the heating fluid;     -   a first heat exchanger;     -   a first fluid circuit connecting the storage reservoir with the         first heat exchanger for carrying heating fluid from the storage         reservoir, through the first heat exchanger and back to the         storage reservoir; and     -   a second fluid circuit connecting an inlet for mains fluid to an         outlet for heated mains fluid via the first heat exchanger, the         second fluid circuit including a portion that passes through an         exhaust path of burner heat expelled after heating of the         heating fluid,     -   wherein the first heat exchanger is adapted to exchange heat         between said heating fluid and said mains fluid.

The first heat exchanger is preferably located outside the storage reservoir. The first heat exchanger preferably includes a first pipe-in-pipe heat exchanger. The first pipe-in-pipe heat exchanger preferably includes a first thermally conductive pipe for containing the heating fluid, said first thermally conductive pipe being located within a second thermally conductive pipe for containing the mains fluid. Alternatively, the first thermally conductive pipe is adapted to carry the mains fluid and the second thermally conductive pipe is adapted to carry the heating fluid. The first heat exchanger preferably includes a plurality of the first and second thermally conductive pipes, with each said first thermally conductive pipe being located within a respective said second thermally conductive pipe. The first thermally conductive pipes are preferably connected in parallel and the second thermally conductive pipes preferably loop backward and forward through the first heat exchanger. The second thermally conductive pipes preferably include a first group of pipes connected in parallel to a second group of pipes.

The first heat exchanger is preferably adapted to be heated by combustion products generated by the burner. The burner is preferably a gas burner and the first heat exchanger is preferably located above the gas burner, such that combustion products from the gas burner rise toward the first heat exchanger. The first heat exchanger preferably includes an array of fins for absorbing heat from the combustion products and transferring said heat to the mains fluid and/or heating fluid in said first heat exchanger.

A second heat exchanger, for exchanging heat between the heating fluid and the mains fluid, is preferably provided downstream of the first heat exchanger. The second heat exchanger is preferably located below the first heat exchanger, such that combustion products passing the second heat exchanger rise toward the first heat exchanger to heat the first heat exchanger. The second heat exchanger preferably includes a second pipe-in-pipe heat exchanger. The second pipe-in-pipe heat exchanger preferably includes a third thermally conductive pipe for containing the mains fluid, said third thermally conductive pipe being located within a fourth thermally conductive pipe for containing the heating fluid. Alternatively, the third thermally conductive pipe is adapted to carry the heating fluid and the fourth thermally conductive pipe is adapted to carry the mains fluid. The second heat exchanger preferably includes an array of fins for absorbing heat from the burner and transferring said heat to the mains fluid and/or heating fluid in said second heat exchanger. The second heat exchanger preferably includes a plurality of the third and fourth thermally conductive pipes, with each said third thermally conductive pipe being located within a respective said fourth thermally conductive pipe. The third thermally conductive pipes are preferably connected in parallel and the fourth thermally conductive pipes preferably loop backward and forward through the second heat exchanger. The fourth thermally conductive pipes are preferably connected in series. The second fluid circuit preferably includes a portion that bypasses the second heat exchanger.

A third heat exchanger, for exchanging heat between the heating fluid and the mains fluid, is preferably provided downstream of the first heat exchanger, and more preferably also downstream of the second heat exchanger. The third heat exchanger is preferably a pipe-in-pipe heat exchanger. More preferably, the third heat exchanger includes a fifth thermally conductive pipe for containing the mains fluid, said fifth thermally conductive pipe being located within a sixth thermally conductive pipe for containing the heating fluid. Alternatively, the fifth thermally conductive pipe is adapted to carry the heating fluid and the sixth thermally conductive pipe is adapted to carry the mains fluid.

The first fluid circuit is preferably defined in part by a first fluid conduit extending from the storage reservoir through the first heat exchanger, through the second heat exchanger, and back to the storage reservoir. The first fluid circuit is preferably defined in part by a third fluid conduit extending from the storage reservoir through the third heat exchanger, through the second heat exchanger and back to the storage reservoir. The third fluid conduit preferably intersects the first fluid conduit upstream of the second heat exchanger.

A first valve is preferably provided in the first fluid conduit for selectively preventing flow of the heating fluid through the first heat exchanger. The first valve is preferably located between the first heat exchanger and the intersection with the third fluid conduit. The first valve is preferably a solenoid valve. A second valve is preferably provided in the first fluid conduit for selectively preventing flow of the heating fluid from the storage reservoir via the first fluid conduit. The second valve is preferably located between the storage reservoir and the first heat exchanger. The second valve is preferably a solenoid valve.

An expansion valve is preferably provided between the second fluid circuit and the first fluid circuit to allow mains fluid to pass from the second fluid circuit into the first fluid circuit when pressure within the second fluid circuit increases to a predetermined level.

A first pump is preferably provided for pumping heating fluid through the first fluid circuit.

A second outlet is preferably provided in the first fluid circuit to allow for supply of the heating fluid to a fourth heat exchanger for exchanging heat between the heating fluid and fluid in an external hydronic heating circuit. The second outlet is preferably provided in a fourth fluid conduit that branches off from the first conduit downstream of the second heat exchanger. A second inlet is preferably provided in the first fluid circuit to allow for the return of heating fluid from the fourth heat exchanger. The second inlet preferably feeds into a fifth fluid conduit that feeds into the first heat exchanger. A third valve is preferably provided for selectively disconnecting flow of heating fluid through the fifth fluid conduit. The third valve is preferably a solenoid valve. A second pump is preferably provided in the fourth or fifth conduits for pumping heating fluid through the fourth and fifth fluid conduits and the fourth heat exchanger. The fourth heat exchanger is preferably a plate heat exchanger.

A valve is preferably provided for selectively controlling the supply of fuel to the burner.

A plurality of temperature and flow sensors are preferably provided for sensing the temperature and flow of fluids at various locations within the fluid heater. A controller is preferably responsive to the sensors for controlling flow of fluid through the fluid heater for safety and/or to allow the fluid heater to operate in various modes. A first temperature sensor is for sensing the temperature of the heating fluid adjacent its point of return to the storage reservoir. A second temperature sensor is provided for sensing the temperature of the heating fluid adjacent its point of return to the storage reservoir. A third temperature sensor is provided for sensing the temperature of the heating fluid downstream of the second heat exchanger. A first flow sensor is provided for sensing the flow of mains fluid adjacent the first inlet. The first flow sensor is preferably a thermal sensor, which senses flow based on the relative temperatures of the mains fluid and the heating fluid. The first flow sensor is preferably located between the mains fluid circuit and the heating circuit adjacent the first inlet. The controller is preferably also responsive to controls of the hydronic heating circuit.

The controller is preferably adapted to close the first valve in response to the first flow sensor sensing flow of mains fluid, to thereby prevent heating fluid from flowing through the first heat exchanger. The controller is preferably adapted to actuate the burner in response to the first flow sensor sensing flow of mains fluid and the first temperature sensor indicating a temperature below a first predetermined value, preferably of approximately 82° C. The controller is preferably adapted to deactivate the burner in response to the first flow sensor sensing flow of mains fluid and the first temperature sensor indicating a temperature above a second predetermined value, preferably of around 85° C.

The controller is preferably adapted to open the first valve in response to the first flow sensor sensing a stoppage of mains fluid flow, to thereby allow heating fluid to flow through the first heat exchanger to scavenge residual heat in the first heat exchanger. The controller is preferably adapted to actuate the burner in response to the first flow sensor sensing a stoppage of mains fluid flow and the second temperature sensor indicating a temperature below a third predetermined value, preferably of around 65° C. The controller is preferably adapted to deactivate the burner in response to the first flow sensor sensing a stoppage of mains fluid flow and the second temperature sensor indicating a temperature above a fourth predetermined value, preferably of around 80° C.

The controller is preferably adapted to actuate the first pump and the second pump in response to the hydronic heating system being activated. The controller is preferably adapted to open the third valve in response to the hydronic heating system being activated. The controller is preferably adapted to close the second valve in response to the hydronic heating system being activated. The controller is preferably adapted to close the third valve and deactivate the second pump if the mains water temperature at the first outlet falls below a predetermined value.

An inlet to the first fluid circuit from the storage reservoir is preferably spaced apart from an outlet from the first fluid circuit into the storage reservoir. More preferably, the inlet and outlet are located at diametrically opposite sides of the storage reservoir.

The fluid heater preferably includes an insulated housing.

In a third aspect, the present invention provides a method for heating fluid, said method comprising the steps of:

-   -   providing a supply of heating fluid;     -   heating the heating fluid;     -   providing mains fluid;     -   heating the mains fluid by exchanging heat between said heating         fluid and said mains fluid; and     -   further heating the mains fluid using residual heat from the         step of heating the heating fluid.

The method preferably includes the further step of reheating the heating fluid if the temperature of the heating fluid falls below a predetermined lower limit. The method preferably includes the further step of stopping the heating step if the temperature of the heating fluid reaches a predetermined upper limit. The method preferably includes the step of changing the upper and lower limits based on whether mains fluid is flowing.

The method preferably includes the additional step of heating fluid in an external hydronic heating circuit by exchanging heat between the heating fluid in the first fluid circuit and fluid in the hydronic heating circuit. The exchanging of heat between the heating fluid in the first fluid circuit and fluid in the hydronic heating circuit is preferably stopped if a temperature of the heating fluid in the first fluid circuit falls below a predetermined lower limit.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, by way of examples only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a first preferred embodiment of a water heater, showing relative water flow paths in a mains (domestic) water heating mode;

FIG. 2 is a schematic view of the heater, showing the relative water flow paths in a maintenance heating mode;

FIG. 3 is a schematic view of the heater, showing the relative water flow paths in a hydronic heating mode;

FIG. 4 is a schematic view of the heater with its front cover removed;

FIG. 5 is a schematic cross-sectional view, taken along line A-A in FIG. 4, showing the flow paths of air and combustion products in a mains (domestic) water heating mode;

FIG. 6 is a schematic cross-sectional view, taken along line B-B in FIG. 4;

FIG. 7 is a schematic perspective rear view of the water heater chassis, showing air channels mounted on the back of the chassis and the top and bottom storage reservoir clamps;

FIG. 8 is a schematic perspective rear view of the heating fluid storage reservoir;

FIG. 9 is a schematic perspective rear view of the chassis with the storage reservoir fitted in position, showing the air channels mounted on the back of the chassis and the top and bottom storage reservoir clamps;

FIG. 10 is a schematic perspective front view of the combustion chamber (second) heat exchanger, with fan mounted on the bottom and flue connecting duct mounted on the top, and also showing the upper (first) heat exchanger;

FIG. 11 is a schematic perspective front view of the combustion chamber (second) heat exchanger, with fan mounted on the bottom and flue connecting duct mounted on the top, connected to the upper (first) heat exchanger and third pipe-in-pipe heat exchanger, which forms a sub-assembly;

FIG. 12 is a schematic perspective front view of the chassis and storage reservoir;

FIG. 13 is a schematic perspective front view of the complete chassis sub-assembly, incorporating the three heat exchangers, the circulation pump and gas valve mounted in position on the chassis, and with the storage reservoir mounted on the back of the chassis (not shown);

FIG. 14 is a schematic exploded cross-sectional view, taken along line C-C of FIG. 5, showing the fitting of the complete chassis sub-assembly to the insulated housing;

FIG. 15 is a schematic cross-sectional view, taken along line C-C of FIG. 5, showing the complete chassis sub-assembly in position inside the insulated housing, but without the top securing insulation and front cover fitted;

FIG. 16 is a schematic exploded cross-sectional view, taken along line C-C of FIG. 5, showing the fitting of the top securing insulation sections and the front cover;

FIG. 17 is a schematic cross-sectional view, taken along line C-C of FIG. 5;

FIG. 18 is a schematic view of the water heater showing the filling/priming flow path of mains water entering the de-oxygenated heating water circuit;

FIG. 19 is a circuit drawing of the water heater's electronic control system;

FIG. 20 is a schematic view of a second embodiment of a water heater;

FIG. 21 is a schematic cross-sectional view, taken along line A-A in FIG. 20, showing the flow paths of air and combustion products in mains (domestic) water, hydronic, and maintenance heating modes, when the burner is activated;

FIG. 22 is a schematic view of the water heater of FIG. 20, showing mains water flow paths in a mains (domestic) water heating mode;

FIG. 23 is a schematic view of the heater of FIG. 20, showing the deoxygenated water flow paths in a maintenance heating mode, with the second (mains) fluid circuit omitted for ease of understanding;

FIG. 24 is a schematic view of the heater of FIG. 20, showing the deoxygenated water flow paths in a hydronic heating mode, with the second (mains) fluid circuit omitted for ease of understanding;

FIGS. 25-27 schematically illustrate the filling/priming flow paths through the heater of FIG. 20;

FIG. 28 is a front elevational view of the third heat exchanger of the heater of FIG. 20;

FIG. 29 is a side elevational view of the third heat exchanger;

FIG. 30 is a top plan view of the third heat exchanger;

FIG. 31 is a cross-sectional view of the third heat exchanger, taken along line 31-31 of FIG. 28;

FIG. 32 is a cross-sectional view of the third heat exchanger, taken along line 32-32 of FIG. 30;

FIG. 33 is a cross-sectional view of the third heat exchanger, taken along line 32-32 of FIG. 30, showing a flow path of mains water therethrough;

FIG. 34 is an enlargement of FIG. 31;

FIG. 35 is a wiring diagram of the heater of FIG. 20 when in a mains (domestic) water heating mode and with the burner on and mains water flowing through the unit;

FIG. 36 is a wiring diagram of the heater of FIG. 20 when in a maintenance mode, with the burner off and the de-oxygenated water in the storage reservoir hot;

FIG. 37 is a wiring diagram of the heater of FIG. 20 when in a maintenance mode, with the burner on and reheating the storage reservoir with no mains water flowing through the unit; and

FIG. 38 is a schematic drawing of an overflow outlet of the heater of FIG. 20.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1-3 of the drawings, there is shown a first embodiment of a water heater 100 comprising a storage reservoir 1 for storing de-oxygenated water for use as a heating fluid. The pressure within the storage reservoir is atmospheric. The storage reservoir includes an overflow 37. The heater also includes a combustion chamber 8 housing a gas burner 5 for heating the de-oxygenated water. A first heat exchanger 11, for exchanging heat between the de-oxygenated water and mains water, is located outside of the storage reservoir 1. A first fluid circuit 200 connects the storage reservoir 1 with the first heat exchanger 11 for carrying de-oxygenated water from the storage reservoir 1, through the first heat exchanger 11 and back to the storage reservoir 1. The inlets 201, 202 to the first fluid circuit 200 from the storage reservoir 1 are spaced apart from the outlet 203 from the first fluid circuit 200 into the storage reservoir 1. The de-oxygenated water within the first fluid circuit 200 is recycled during operation of the water heater 100. A second fluid circuit 300 connects a first inlet 19 for mains water to a first outlet 27 for heated mains water via the first heat exchanger 11. The mains water and de-oxygenated water are not mixed during operation of the water heater 100.

The first heat exchanger 11 includes a pipe-in-pipe heat exchanger 12 having a plurality of first thermally conductive pipes 12 a for containing the de-oxygenated water. The first thermally conductive pipes 12 a are each located within a respective second thermally conductive pipe 12 b for containing the mains water. The first thermally conductive pipes 12 a are connected in parallel and the second thermally conductive pipes 12 b loop backward and forward through the first heat exchanger 11. The second thermally conductive pipes 12 b include a first (upper) group of pipes connected in parallel to a second (lower) group of pipes. The first heat exchanger 11 is located above the gas burner 5, such that combustion products from the gas burner 5 rise toward the first heat exchanger to heat the first heat exchanger 11. The combustion products enter the first heat exchanger 11 at a temperature in the range of approximately 100° C. to 160° C., dependent on the water heater's temperature settings and burner fuel input rate. The first heat exchanger 11 also includes an array of fins 12 c for absorbing heat from the burner combustion products and transferring the absorbed heat to the mains water and/or de-oxygenated water in the first heat exchanger 11.

A second heat exchanger 101, for exchanging heat between the de-oxygenated water and the mains water, is provided downstream of the first heat exchanger 11. The second heat exchanger 101 is located below the first heat exchanger 11, such that combustion products passing the second heat exchanger 101 rise toward the first heat exchanger 11 to heat the first heat exchanger 11. The second heat exchanger 101 also includes a pipe-in-pipe heat exchanger 10 having a plurality of third thermally conductive pipes 10 a for containing the mains water. The third thermally conductive pipes 10 a are each contained within a respective fourth thermally conductive pipe 10 b for containing the de-oxygenated water. The second heat exchanger 101 also includes an array of fins 10 c for absorbing heat from the burner combustion products and transferring the heat to the mains water and/or de-oxygenated water in the second heat exchanger 101. The third thermally conductive pipes 10 a are connected in parallel and the fourth thermally conductive pipes 10 b loop backward and forward through the second heat exchanger 101. The fourth thermally conductive pipes 10 b are connected in series.

The second fluid circuit 300 includes a portion 301 that bypasses the second heat exchanger 101. The bypassing portion 301 re-joins the remainder of the second fluid circuit 300 upstream of a third heat exchanger 25, at a position 26.

The third heat exchanger 25 is located downstream of the second heat exchanger 101 and is adapted to exchange heat between the de-oxygenated water and the mains water. The third heat exchanger 25 also includes a pipe-in-pipe heat exchanger having a fifth thermally conductive pipe 25 a for containing the mains water. The fifth thermally conductive pipe 25 a is located within a sixth thermally conductive pipe 25 b for containing hot de-oxygenated water drawn directly from the storage reservoir 1.

The first fluid circuit 200 is defined in part by a first fluid conduit 204 extending from the storage reservoir 1 through the first heat exchanger 11, through the second heat exchanger 101, and back to the storage reservoir 1. The first fluid circuit 200 is defined in part by a third fluid conduit 205 extending from the storage reservoir 1 through the third heat exchanger 25, through the second heat exchanger 101 and back to the storage reservoir 1. The third fluid conduit 205 intersects the first fluid conduit 204 upstream of the second heat exchanger 101.

A first solenoid valve 14 is provided in the first fluid conduit 204, between the first heat exchanger 11 and the intersection with the third fluid conduit 205, for selectively preventing flow of the de-oxygenated water through the first heat exchanger 11. A second valve 35 is provided in the first fluid conduit 204, between the storage reservoir 1 and the first heat exchanger 11, for selectively preventing flow of the de-oxygenated water from the storage reservoir 1 via the first fluid conduit 204. An expansion valve 36 is provided between the second fluid circuit 300 and the first fluid circuit 200 to allow mains fluid to pass from the second fluid circuit 300 into the first fluid circuit 200 when pressure within the second fluid circuit 300 increases to a predetermined level.

A first pump 28 is provided for pumping de-oxygenated water through the first fluid circuit 200. A drain tap 47 is also provided in the first fluid circuit to allow drainage of the de-oxygenated water for servicing.

A second outlet 30 is provided in the first fluid circuit 200 to allow for supply of the de-oxygenated water to a fourth plate-type heat exchanger 32 for exchanging heat between the de-oxygenated water and fluid in an external hydronic heating circuit (not shown). The second outlet 30 feeds into in a fourth fluid conduit 206 that branches off from the first conduit 204 downstream of the second heat exchanger 101. A second inlet 30 a is provided in the first fluid circuit 200 to allow for the return of de-oxygenated water from the fourth heat exchanger 32. The second inlet 30 a feeds into a fifth fluid conduit 207 that feeds into the first heat exchanger 11. A third solenoid valve 31 is provided for selectively disconnecting flow of de-oxygenated water through the fifth fluid conduit 207. A second pump 29 is provided in the fifth conduit 207 for pumping de-oxygenated water through the fourth 206 and fifth 207 fluid conduits and the fourth heat exchanger 32.

A plurality of temperature and flow sensors are provided for sensing the temperature and flow of fluids at various locations within the water heater 100. A controller 38 is responsive to the sensors for controlling flow of fluid through the water heater 100 for safety and to allow the water heater to operate in various modes. A first temperature sensor 39 is soldered to the first fluid conduit 204 adjacent its point of return to the storage reservoir 1 for sensing the temperature of the de-oxygenated water. A second temperature sensor 40 is also soldered to the first fluid conduit adjacent its point of return to the storage reservoir 1 for sensing the temperature of the de-oxygenated water. A third temperature sensor 45 is provided for sensing the temperature of the de-oxygenated water downstream of the second heat exchanger 101. A first flow sensor, in the form of a thermal sensor 41, which senses flow based on the relative temperatures of the mains water and the de-oxygenated water, is located between the mains water circuit 300 and the de-oxygenated water circuit 200, adjacent the first inlet 19, for sensing the flow of mains water. The first flow sensor 41 is soldered to a conduit of the mains fluid heating circuit and to a conduit of the de-oxygenated water circuit. The controller 38 is also responsive to controls of the hydronic heating circuit (not shown). Should the water heater 100 ever overheat due to a component failure, the controller 38 receives a signal from the third temperature sensor 45 and fuel to the burner 5 is stopped.

As shown in FIG. 1, the controller 38 is adapted to close the first valve 14 in response to the first flow sensor 41 sensing flow of mains water, to thereby prevent de-oxygenated water from flowing through the first heat exchanger 101. The controller 38 is adapted to open a fuel valve 23 for supplying fuel to the burner 5 and to actuate the burner 5 in response to the first flow sensor 41 sensing flow of mains water and the first temperature sensor 39 indicating a temperature below a first predetermined value of approximately 82° C. The controller 38 is adapted to close the fuel valve 23 and deactivate the burner 5 in response to the first flow sensor 41 sensing flow of mains water and the first temperature sensor 39 indicating a temperature above a second predetermined value of approximately 85° C.

As shown in FIGS. 2 and 3, the controller 38 is adapted to open the first valve 14 in response to the first flow sensor 41 sensing a stoppage of mains water flow, to thereby allow de-oxygenated water to flow through the first heat exchanger 11 to scavenge residual heat in the first heat exchanger 11. The controller 38 is adapted to open fuel valve 23 and actuate the burner 5 in response to the first flow sensor 41 sensing a stoppage of mains water flow and the second temperature sensor 40 indicating a temperature below a third predetermined value of approximately 65° C. The controller 38 is adapted to close the fuel valve 23 and deactivate the burner 5 in response to the first flow sensor 41 sensing a stoppage of mains water flow and the second temperature sensor 40 indicating a temperature above a fourth predetermined value of approximately 80° C.

As shown in FIG. 3, the controller 38 is adapted to actuate both the first pump 28 and the second pump 29, and to open the third valve 31 and close the second valve 35, in response to the hydronic heating system (not shown) being activated. The controller 38 is adapted to close the third valve 31 and deactivate the second pump 29 if the mains water temperature at the first outlet 27 falls below a user's preset desired hot water supply temperature.

As shown in FIGS. 4-13, the three pipe-in-pipe heat exchangers 11, 101, 25, the storage reservoir 1 and all other components of the water heater 100 are fixed to a central chassis 3, which is suspended inside a housing 4. Insulation 42 and 43 is fitted to the housing 4 to reduce heat losses. The insulation also supports the chassis 3 and prevents the internal water heater components from making any contact with the housing 4. Accordingly, the potential for any heat losses through the housing 4 is reduced.

The chassis 3 is fitted with air channels 44, which provide an air gap to allow heat from the back of the combustion chamber 8, and first heat exchanger 11 to heat the air in the gap via contact with the chassis 3. The heated air is subsequently drawn by a forced draft fan 6 located in the combustion chamber 8, whilst the burner 5 is on, into the combustion chamber 8 and forced up and through the first 11 and second 101 heat exchangers. As shown in FIGS. 4, 7, 9, 12 and 13, the chassis 3 has vent holes 46 in its top and base to enhance the thermal rise of heat whilst the burner 5 is actuated.

As shown in FIG. 5, the housing 4 has an air inlet vent 7. The air inlet vent 7 is orientated vertically and forms an air heat trap which reduces the potential for any heat losses during maintenance mode. An outlet duct 15 is also provided for the exhaust of cooled combustion products. The outlet duct 15 is also orientated vertically to form an air heat trap, which reduces the potential for heat loss.

The operation of the water heater 100 shall now be described.

In mains water heating mode, as shown in FIG. 1, mains water enters the water heater 100 via inlet 19. In response to the first flow sensor 41 sensing flow of mains water, the controller 38 opens the fuel valve 23 and ignites the burner 5. The controller 38 also closes solenoid valve 14 to prevent de-oxygenated water from flowing through the first heat exchanger 11. The mains water passes through the first heat exchanger 11, where it is heated by exchanging heat with hot combustion products rising through the first heat exchanger 11 and by exchanging heat with the relatively hot, stationary, de-oxygenated water in pipes 12 a of the heat exchanger 12. Because the de-oxygenated water in the first heat exchanger 11 is stationary, and as to the thermally conducting pipes 12 a carrying the de-oxygenated water are inside the pipes 12 b carrying the mains water, the mains water absorbs most of the heat from combustion products rising through the first heat exchanger 11.

The partially heated mains water then passes into the second heat exchanger 101, where it is heated by exchanging heat with the relatively hot de-oxygenated water therein. Some of the mains water also bypasses the second heat exchanger 101 via portion 301 of the second fluid circuit 300. The mains water passing through the second heat exchanger 101 and through portion 301 continues on through to the third heat exchanger 25, where the mains water is further heated by exchanging heat with the de-oxygenated water therein. The third heat exchanger 25 provides the largest temperature rise for the mains water during its passing through the water heater 100. However, the other heat exchangers 11, 101 also impart significant heat to the mains water. After exiting the third heat exchanger 25, the heated mains fluid passes out of the water heater 100 via outlet 27.

After a mains hot water tap (not shown) is closed and the water heater 100 ceases being called upon to supply hot mains water, controller 38 opens solenoid valve 14 to allow the de-oxygenated water to cycle through the first heat exchanger 11 to scavenge any residual heat in still rising combustion products. The burner 5 also remains on, until the sensor 40 indicates a de-oxygenated water temperature of 80° C., to reinstate heat lost from the de-oxygenated water during heating of the mains water.

As will be appreciated, at the completion of mains water heating mode, mains water trapped in the second fluid circuit 300, after a tap (not shown) downstream of the water heater 100 is closed, will continue to heat and expand. As the trapped mains water expands, pressure in the second fluid circuit 300 is released by allowing small injections of mains water into the first fluid circuit 200 through expansion valve 36. These small injections of mains water also serve to top up the water level in the first fluid circuit 200 to account for any losses. If the water level in the first fluid circuit 200 becomes too large, however, it simply overflows via overflow 37 of the storage reservoir 1.

In maintenance heating mode, as shown in FIG. 2, the water heater 100 is not being called upon to supply hot mains water and the controller 38 opens solenoid valve 14 in response to sensor 41 sensing no flow of mains water. The de-oxygenated water is continuously cycled through the first 11, second 101 and third 25 heat exchangers. The burner 5 is periodically actuated and deactivated by the controller 38 in response to feedback from sensor 40 to maintain the de-oxygenated water at a temperature of between 65 and 80° C.

FIG. 3 shows a hydronic heating mode of the water heater 100. In response to the hydronic heating mode being actuated, the controller 38 closes solenoid valve 35 to prevent de-oxygenated water from flowing from the storage reservoir 1 directly into the first heat exchanger 11. The controller 38 also opens solenoid valve 31 and actuates the second pump 29.

The closed solenoid valve 35 ensures that only the returned (cooled) de-oxygenated water from the fourth heat exchanger 32 is circulated though the first heat exchanger 11. This ensures that the temperature differential between the combustion products in the first heat exchanger 11 and the de-oxygenated water (which has been cooled by the hydronic system in plate heat exchanger 32) is the largest achievable and consequently the highest possible combustion/thermal efficiency can be maintained during hydronic heating mode.

The de-oxygenated water is continuously cycled through the first 11, second 101 and third 25 heat exchangers. The burner 5 is periodically actuated and deactivated by the controller 38 in response to feedback from sensor 40 to maintain the de-oxygenated water at a temperature of between 65 and 80° C.

The controller 38 is also fitted with a mains water priority sensor (not shown), which automatically disconnects supply to the hydronic heating system should the mains water outlet temperature ever drop below the user's desired preset hot water temperature. The cut-off setting for the hydronic heating system is also adjustable to suit the operating temperature of the hydronic system and ensures that the domestic mains water is always given priority over the hydronic heating system should the unit not be able to supply sufficient energy to run both functions.

The expansion valve 36 operates in the same manner in the hydronic heating mode as in the maintenance heating mode.

FIG. 19 shows a circuit drawing of the water heater's electronic control system. In addition to a number of the abovementioned control components, FIG. 19 also shows mains power MP, a plurality of switches Sw1, Sw2, Sw3, Sw4 and Sw5, an air pressure switch Sw_(AP), a flame proof ignitor I, and a hydronic heating thermostat T_(H).

Ventilation of the water heater 100 will now be described.

As shown in FIG. 5, during operation of the burner 5, the controller 38 actuates the forced draft fan 6 to draw combustion air into a space around the combustion chamber 8 and then through the fan 6 where the combustion air is mixed with burner fuel for ignition. This flow path of the incoming air ensures that the exterior surfaces of both the combustion chamber 8 and the fins 12 c of the first heat exchanger 11 are air cooled.

As shown in FIGS. 7, 9, 12 and 13, cooler air inside the bottom of the housing 4 is drawn into vent holes 46 in the bottom of the chassis 3, rises up between the chassis 3 and the storage reservoir 1 and exits into the top of the housing 4 via vent holes 46 in the top of the chassis 3. This air movement prevents the combustion chamber 8 and the storage reservoir 1 from overheating during combustion. The heated cooling air is then re-directed back into the combustion chamber 8, and through the second 101 heat exchanger and then through the first heat exchanger 11, via the forced draft fan 6.

As shown in FIG. 5, high temperature combustion products 9, shown as vertical arrows, are forced up to make contact with the second heat exchanger 101 at the top of the combustion chamber 8. The high temperature combustion products 9 heat exchange with the de-oxygenated water in the outer pipe 10 b, which is always flowing when the burner 5 is on, and the mains water in the inner pipe 10 a of the second heat exchanger 101, which only flows when hot mains water is being consumed.

It will be appreciated that the combustion products 13 that have passed through the second heat exchanger 101, and have heat exchanged with the de-oxygenated water in the outer pipe 10 b, are now significantly cooler. These cooler combustion products 13 then rise and pass through the first heat exchanger 11, where they heat exchange predominantly with the mains water in the outer thermally conductive pipes 12 b.

As shown in FIG. 5, after the combustion products 13 have passed through the first heat exchanger 11, they enter flue outlet duct 15, which forces them vertically down to the flue outlet 16, which directs the cooled combustion products 17 horizontally out and away from a front cover 18 of the housing 4.

As the combustion products 13 cool in the first heat exchanger 11, they form condensation droplets on the surface of the vertically orientated heat exchanging fins 12. As shown in FIGS. 1-3 and 5, the droplets flow vertically down the fins 12 of the first heat exchanger 11 and collect in a condensation bath 20 at the base of the first heat exchanger 11. The condensation then flows through a drain outlet 21 in the base of the condensation bath 20 and out of the water heater 100 via a drain pipe 22.

The condensation bath 20 is positioned so that it is not in contact with any hot surfaces or in the path of any high temperature combustion products which ensures that the condensate will not re-evaporate or form a vapour. The condensate liquid drains from the water heater 100 at a relatively low temperature.

Filling/priming of the water heater 100 will now be described.

After the water heater 100 has been installed and all the water and gas pipes are connected, the installer opens the expansion valve 36 by pulling on a lever at the top of the valve 36. As shown in FIG. 18, this injects mains pressure water into the de-oxygenated water circuit 200 as shown by the arrows, which demonstrate the filling/priming flow path of the mains water into the de-oxygenated water circuit 200. As can be seen in FIG. 18, the mains water injected from the expansion valve 36 flows into the de-oxygenated water pipe 25 b of the third heat exchanger 25 in a reverse direction to the normal (pump) flow direction.

The mains water flows through the third heat exchanger 25 and the second heat exchanger 101 and forces all the air out of the fluid conduits 204 and 200 as it enters the storage reservoir 1. The flowing mains water fills the storage reservoir 1 until the installer notices that water is flowing out of the overflow outlet 37, which indicates that the storage reservoir 1 is full and the third heat exchanger 25 is primed and has no air inside it.

At this point, the installer connects the power to the water heater 100 which automatically starts a preset/programmed cold start sequence. The cold start sequence is preset into the controller 38 and initiates the following purging sequence of the water heater 100 prior to an ignition sequence for lighting the burner 5.

In the cold start mode the controller 38 opens solenoid valve 14 and activates pump 28 for a set period of time (between 3 and 10 minutes) to enable priming of the first heat exchanger 11 and remove any air that is still present in the de-oxygenated pipes 12 a of the first heat exchanger 11.

The configuration of the storage reservoir 1 and the first heat exchanger 11 is such that it causes the water in the storage reservoir 1 to automatically siphon through the first heat exchanger 11. Also, with the aid of the pump 28, during the cold start purging period, the water stored in the storage reservoir 1 automatically purges the first heat exchanger 11 of air and fills it with the water from storage reservoir 1.

The ignition sequence for burner 5 commences after the cold start purging sequence is completed. In the ignition sequence, the combustion fan 6, the gas valve 23 and the circulation pump 28 are simultaneously actuated and the water heater 100 reverts to its controller 38 to govern all operational modes, as shown in FIG. 19.

The water heater 100 can be manually reset to the cold start mode, by an activation switch on the electronic controller 38, to re-prime the water heater 100, for example after drainage of the de-oxygenated water circuit 200 or storage reservoir 1 for inspection or repair.

The above described embodiment provides the following benefits:

-   -   The pipe-in-pipe configuration allows for both the mains water         and de-oxygenated water to be utilised as a conduit medium for         heat transfer, which provides a high efficiency performance in         mains water heating, maintenance and hydronic heating modes.     -   The pipe-in-pipe configuration enables the water heater to         suffer virtually no negative effect in terms of longevity if the         water supply is corrupted with impurities, as the mains water         circuit 300, which potentially contains impurities, is separated         from the de-oxygenated water circuit 200 and consequently the         potential for scaling is significantly reduced.     -   The de-oxygenated water in the water heater 100 is not replaced         and is constantly re-circulated between the storage reservoir 1         and the heat exchangers 11, 101, 25. Accordingly, minimal         topping up of the de-oxygenated water is required, which thereby         reduces the probability of the de-oxygenated water being         contaminated.     -   In mains water heating mode, the large temperature differential         between the flowing cold mains water and the combustion products         13 in the first heat exchanger 11 provides a combustion/thermal         efficiency of approximately 95-98%, dependent on the mains water         inlet temperature.     -   In maintenance mode, the temperature differential between the         recirculating de-oxygenated water and the combustion products 13         is not as large as during mains water heating mode. However,         there is still a large increase in efficiency contributed via         the de-oxygenated water flowing through the first heat exchanger         11. Accordingly, the combustion/thermal efficiency in         maintenance mode is between 90% and 94%, dependent on the         temperature setting of de-oxygenated water in the storage         reservoir 1.     -   In hydronic heating mode, by isolating and delivering the         returned (cooled) de-oxygenated water from the return line 207         from the fourth heat exchanger 32 directly into the first heat         exchanger 11, a large temperature differential is created with         the combustion products 13. This large temperature differential         can again produce a high efficiency performance. However, the         operating temperature of the heating system has a direct bearing         on the efficiency of the water heater 100 in hydronic heating         mode. If the hydronic heating system temperature is set above         80° C., the water heater will provide a thermal/combustion         efficiency close to 90%. If the hydronic heating system is set         under 60-50° C., or the returning water from the hydronic         heating system is lower than 45-50° C., the water heater 100 can         provide high efficiency combustion/thermal performance         (condensing) of between 90 and 95%.     -   The de-oxygenated water in pipes 12 a of the first heat         exchanger 12 remains static, when the water heater 100 is in a         water heating mode, to ensure that the cold mains water can         recover/absorb the bulk of the heat energy from the hot         combustion products 13.     -   The ability of the water heater 100 to continually cool the         first heat exchanger 11, in circumstances whereby the burner 5         is cycled on and off, by continuously cycling cold mains water         through the first heat exchanger 11.     -   If the water heater 100 is functioning as a domestic water         heater and the flow rate (energy requirement) is lower than the         water heater's capacity, the burner 5 will cycle on and off to         save fuel.     -   The air flow path inside the water heater 100 ensures that heat         losses from the combustion chamber 8 are re-directed back to the         combustion chamber 8, where the heat is recovered via the first         11 and second 101 heat exchangers.     -   The inlet and outlet ports 201, 202, 203 from the storage         reservoir 1 all penetrate via the reservoir's horizontal top         lid, so as not to contact de-oxygenated water within the         reservoir 1. By positioning the exit and entry points of the         inlet and outlets 201, 202, 203 in the air gap 2 at the top of         the reservoir 1, the potential for leaks to occur is reduced.         This design also enables a variety of alternate materials to be         used for the reservoir 1.     -   The small 2° C. differential between the upper cut out         temperature setting of sensor 40 and the cut-in temperature         setting of sensor 39 ensures that the burner 5 responds almost         immediately at the initiation of mains water heating mode.     -   The 3° C. differential between the cut-in and cut-out         temperature settings of sensor 39, combined with the         recirculating of the hot deoxygenated water in the storage         reservoir 1, ensures a consistent temperature delivery during         domestic mains water heating mode during cycling of the burner         5.     -   The relatively large 15° C. differential between the cut-in and         cut-out temperature settings of sensor 40 ensures that the water         heater 100 performs only one recovery burn sequence after a         mains domestic water draw off, a hydronic heating draw off, or a         maintenance burn.     -   The use of dual sensors 39 and 40 also prevent nuisance burns         (stair casing), which can occur in single thermostat systems.     -   As flow sensor 41 senses mains water flow by a thermal reading         and not a flow reading, it can operate at any mains domestic         water flow rate or pressure.     -   When the burner 5 is off, the heater 100 is able to maintain a         consistent hot mains water output due to the heat energy stored         in the relatively hot de-oxygenated water stored in the         reservoir 1, which is pumped through the various heat exchangers         11, 101, 25 to heat the mains water until the burner 5 has         activated.     -   The mixing of de-oxygenated water returning to the reservoir 1         with de-oxygenated water already in the reservoir 1 is enhanced         by the opposing entry and exit ports of the reservoir 1 and the         velocity of the circulation pump 28. This mixing ensures that         the reservoir 1 supplies de-oxygenated water of a consistent         temperature, which reduces temperature fluctuations in the hot         flowing mains water whilst burner 5 is cycling on and off in         mains water heating mode.

It will be appreciated that many modifications may be made to the first embodiment. For example:

-   -   in the first heat exchanger, the first thermally conductive pipe         can be adapted to carry the mains water and the second thermally         conductive pipe can be adapted to carry the de-oxygenated water;     -   in the second heat exchanger, the third thermally conductive         pipe can be adapted to carry the de-oxygenated water and the         fourth thermally conductive pipe can be adapted to carry the         mains water;     -   in the third heat exchanger, the fifth thermally conductive pipe         can be adapted to carry the de-oxygenated water and the sixth         thermally conductive pipe can be adapted to carry the mains         water;     -   the gas burner can be replaced with an electric burner, an oil         burner or another type of fuel burner;     -   the fourth heat exchanger can be a radiator panel, air convector         (forced draft or natural convection), or underfloor/slab heating         coils;     -   the water heater preferably only has one gas input rate, which         is the maximum rate as determined by the burner fuel valve 23         settings. However, alternative gas or oil management systems         (modulating input rates) can be applied to the water heater;     -   the atmospheric pressure storage reservoir can be replaced with         a pressurised storage reservoir governed by a conventional         pressure and temperature relief/safety valve;     -   the upper and lower limits of the various sensors can be         adjusted based on the user's requirements and/or operating         parameters of the water heater (eg. desired hot water output         temperature, cold mains water input temperature); and/or     -   the third heat exchanger 25 can be placed in an exhaust path of         combustion products exhausted from the first heat exchanger 11.

Referring to FIGS. 20 to 38 of the drawings, there is shown a second embodiment of a water heater 400 comprising a storage reservoir 401 for storing de-oxygenated water for use as a heating fluid. The pressure within the storage reservoir is atmospheric. The storage reservoir includes an overflow 402. The heater 400 also includes a combustion chamber 403 housing a gas burner 404 for heating the de-oxygenated water. A first heat exchanger 405, for exchanging heat between the de-oxygenated water and mains water, is located outside of the storage reservoir 401. A first fluid circuit 406 connects the storage reservoir 401 with the first heat exchanger 405 for carrying de-oxygenated water from the storage reservoir 401, through the first heat exchanger 405 and back to the storage reservoir 401, as shown in FIG. 23. The return pipe 407 to the first fluid circuit 406 from the storage reservoir 401 causes the returning de-oxygenated water to mix with the de-oxygenated water inside the storage reservoir 401. The de-oxygenated water within the first fluid circuit 406 is recycled during operation of the water heater 400. As shown in FIG. 22, a second fluid circuit 410 connects a first inlet 411 for mains water to a first outlet 412 for heated mains water via the first heat exchanger 405. The mains water and de-oxygenated water are not mixed during operation of the water heater 400.

The first heat exchanger 405 includes a pipe-in-pipe heat exchanger 413 having a plurality of first thermally conductive pipes 414 a for containing the de-oxygenated water. The first thermally conductive pipes 414 a are each located within a respective second thermally conductive pipe 414 b for containing the mains water. The first thermally conductive pipes 414 a are connected in parallel and the second thermally conductive pipes 414 b loop backward and forward through the first heat exchanger 405. The second thermally conductive pipes 414 b include a first (upper) group of pipes connected in parallel to a second (lower) group of pipes. The first heat exchanger 405 is located above the gas burner 404, such that combustion products from the gas burner 404 rise toward the first heat exchanger to heat the first heat exchanger 405. The combustion products enter the first heat exchanger 405 at a temperature in the range of approximately 100° C. to 160° C., dependent on the water heater's temperature settings and burner fuel input rate.

The first heat exchanger 405 also includes an array of fins 415 for absorbing heat from the burner combustion products and transferring the absorbed heat to the mains water and/or de-oxygenated water in the first heat exchanger 405.

A second heat exchanger 416 is located below the first heat exchanger 405, such that combustion products passing the second heat exchanger 416 rise toward the first heat exchanger 405 to heat the first heat exchanger 405. The second heat exchanger 416 also includes an array of fins 417 for absorbing heat from the burner combustion products and transferring the heat to the de-oxygenated water in the portion of the first fluid circuit 406 passing through the second heat exchanger 416.

A third heat exchanger 409 is immersed in the deoxygenated water in the storage reservoir 401. The third heat exchanger 409 is located downstream, in the second fluid circuit 410, of the first heat exchanger 405. The third heat exchanger 409 is adapted to exchange heat between the de-oxygenated water and the mains water. The third heat exchanger 409 also includes a pipe-in-pipe heat exchanger having a series of third thermally conductive pipes 409 a for containing the mains water. The third thermally conductive pipes 409 a are located within two fourth thermally conductive pipes 409 b, which contain hot de-oxygenated water. The hot de-oxygenated water in the pipes 409 b is drawn directly from the storage reservoir 401, via inlet bleed holes 408 and inlets 420.

An expansion valve 422 is provided between the second fluid circuit 410 and the first fluid circuit 406 to allow mains fluid to pass from the second fluid circuit 410 into the first fluid circuit 406 when pressure within the second fluid circuit 410 increases to a predetermined level.

A first pump 423 is provided for pumping de-oxygenated water through the first fluid circuit 406. A drain tap 424 is also provided in the first fluid circuit 406 to allow drainage of the de-oxygenated water for servicing. The drain tap 424 is connected to expansion valve 422 and, upon opening, allows de-oxygenated water to drain from the first fluid circuit 406 and storage reservoir 401.

As best seen in FIG. 24, an outlet 425 is provided in the storage reservoir 401 to allow for supply of the de-oxygenated water to a fourth plate-type heat exchanger 427, via a third fluid circuit 426, for exchanging heat between the de-oxygenated water and fluid in an external hydronic heating circuit (not shown). An inlet 425 a is provided in the first fluid circuit 406 to allow for the return of de-oxygenated water from the fourth heat exchanger 427. The inlet 425 a feeds into the first fluid circuit 406 upstream from the first heat exchanger 405. A second pump 434 is provided in the third fluid circuit for pumping de-oxygenated water through the first 406 and third 426 fluid conduits and the fourth heat exchanger 427.

Two thermostat sensors 429 and 430 are provided for sensing the temperature and flow of fluids at various locations within the water heater 400. The first thermostat sensor 429 is fitted to the first fluid circuit 406 inside the first fluid conduit 421 downstream from the second heat exchanger 416 and above storage reservoir 401 for sensing the temperature of the de-oxygenated water in the first fluid circuit 406 and storage reservoir 401. A controller (not shown) is responsive to the thermostat sensor 429 for activating and de-activating the burner 404.

The second thermostat sensor 430 is fitted to the second fluid circuit 410 inside the second fluid conduit 431 adjacent, and in physical contact with, the first fluid conduit 421 at a point of return to the storage reservoir 401, for sensing the temperature of the mains water. The controller (not shown) is responsive to the thermostat sensor 430 for activating and deactivating pump 423 upon sensing temperature change in the mains water flowing through the second fluid circuit 410.

A safety cut-out switch 432 is provided for sensing the temperature of the de-oxygenated water downstream of the second heat exchanger 416. Should the water heater 400 ever overheat due to a component failure, the safety cut out switch 432 will stop the supply of fuel to the burner 404.

In response to thermostat sensor 429 sensing a de-oxygenated water temperature below a first predetermined value of approximately 82° C., the controller (not shown) is set to open a fuel valve 433 for supplying fuel to the burner 404 and to actuate the burner 404 and pump 423. In response to thermostat sensor 429 sensing a de-oxygenated water temperature above a second predetermined value of approximately 85° C., the controller is set to close the fuel valve 433, deactivate burner 404 and stop the pump 423.

In response to sensor 430 sensing a mains water temperature below a first predetermined value of approximately 50° C., the controller (not shown) is set to activate pump 423. In response to thermostat sensor 430 sensing a mains water temperature above a second predetermined value of approximately 55° C., the controller (not shown) is set to deactivate pump 423. The controller (not shown) responds to feedback from both sensors 429 and 430 independently.

As shown in FIG. 24, a hydronic heating controller (not shown) is adapted to actuate pump 423, a second pump 434, and a third pump 435 in response to the hydronic heating system (not shown) being activated. The hydronic heating controller is adapted to deactivate both the second pump 434 and third pump 435 if the mains water temperature at the first outlet 412, as shown in FIG. 22, falls below a user's preset desired hot water supply temperature.

As shown in FIGS. 20 and 21, the two pipe-in-pipe heat exchangers 405 and 409, the storage reservoir 401 and all other components of the water heater 400 are fixed to a central chassis 436, which is suspended inside a housing 437. Insulation 438 and 439 is fitted to the housing 437 to reduce heat losses. The insulation 438, 439 also supports the chassis 436 and prevents the internal water heater components from making any contact with the housing 437 using the same principles as shown in FIGS. 14, 15, 16, and 17 of the first embodiment. Accordingly, the potential for any heat losses through the housing 437 is reduced.

The chassis 436 is fitted with air channels 440, which provide an air gap to allow heat from the back of the combustion chamber 403, and first heat exchanger 405 to heat the air in the air gap via contact with the chassis 436. The heated air is subsequently drawn by a forced draft fan 441 connected to the combustion chamber 403, whilst the burner 404 is on, into the combustion chamber 403 and forced up and through the second 416 and first 405 heat exchangers.

As shown in FIG. 21, the housing 437 has an air inlet vent 442. The air inlet vent 442 is orientated vertically and forms an air heat trap which reduces the potential for any heat losses during a maintenance mode. An outlet duct 443 is also provided for the exhaust of cooled combustion products. The outlet duct 443 is also orientated vertically to form an air heat trap, which reduces the potential for heat loss.

The operation of the water heater 400 shall now be described.

In mains water heating mode, as shown in FIGS. 22 and 35, mains water enters the water heater 400 via inlet 411. In response to thermostat sensor 430 sensing a temperature change due to the flow of mains water, the controller (not shown) actuates pump 423 and starts circulating hot de-oxygenated water through the first fluid circuit 406. Prior to entering the water heater 400, the flowing mains water in the second fluid circuit 410 is cold and heat exchanges with the hot de-oxygenated water which is being pumped through both pipe-in-pipe heat exchangers 409 and 413. The flowing mains water is heated by the flowing de-oxygenated water and the de-oxygenated water is accordingly cooled by the flowing mains water. Thermostat sensor 429 senses the resulting drop in de-oxygenated water temperature and causes the controller 500 to open the fuel valve 433 and ignite the burner 404.

The mains water passes through the first heat exchanger 405, where it is heated by exchanging heat with hot combustion products rising through the first heat exchanger 405 and by exchanging heat with the hot, flowing de-oxygenated water in pipes 414 a of the heat exchanger 413. The de-oxygenated water in the first heat exchanger 405 is flowing and also provides heat to the flowing mains water in the first heat exchanger 405. Heat is applied to the flowing mains water from both the hot combustion products and the flowing hot de-oxygenated water inside the first heat exchanger 405.

Because the flowing mains water is cold, it can cool the combustion products flowing through the first heat exchanger 405 to an extent that a high combustion efficiency in the range of 90 to 99% can be achieved, dependent on the ambient temperature and water temperature that the heater 400 is operating in. A combustion efficiency above 90% can create large volumes of condensate in the first heat exchanger 405.

The partially heated mains water then passes into the third heat exchanger 409, where it is heated by exchanging heat with the hot de-oxygenated water that is circulating through the second heat exchanger 416, storage reservoir 401 and third heat exchanger 409.

Both pipe in pipe heat exchangers 409 and 413 provide temperature rise for the mains water during its passing through the water heater 400. After exiting the second heat exchanger 409, the heated mains fluid passes out of the water heater 400 via outlet 412.

After a mains hot water tap (not shown) is closed and the water heater 400 ceases being called upon to supply hot mains water, thermostat sensor 429 allows the de-oxygenated water to continue to cycle through the first fluid circuit 406 and heat exchangers 405 and 409 to scavenge any residual heat in the still rising combustion products from the combustion chamber 403. The burner 404 also remains on, until the thermostat sensor 429 senses a de-oxygenated water temperature of 85° C., to reinstate heat lost from the de-oxygenated water during heating of the mains water. After thermostat sensor 429 senses a de-oxygenated water temperature of 85° C., the controller (not shown) closes the supply of fuel to the fuel valve 433 and the burner 404 shuts down.

The controller (not shown), in response to feedback from thermostat sensor 430, may maintain the pump 423, which circulates the de-oxygenated water through the first fluid circuit 406 after the burner 404 has shut down, until the temperature in the mains water adjacent to the sensor 430 rises to its shut-off setting of approximately 55° C.

The burner 404 may reactivate whilst the mains water is reheating until the thermostat sensor 430 reaches its pre set shut-off temperature setting of approximately 55° C. However, after the thermostat sensor 430 reaches its pre set shut-off temperature setting and has shut down the pump 423, thermostat sensor 429 gains full control of the pump 423 and simultaneously activates and deactivates the pump 423, fuel valve 433 and fan 441 upon feedback from thermostat sensor 429, which only senses the temperature of the de-oxygenated water circulating through the first fluid circuit 406.

The simultaneous shutting down of pump 423, fuel valve 433 and fan 441 prevents any repeat ignition and burn sequences, which can occur due to the differential temperature rise that occurs across the second heat exchanger 416. Because pump 423 stops simultaneously with the burner 404 (after thermostat sensor 430 has reached its shut-off temperature setting), the thermostat sensor 429 is responding to and reading the hottest de-oxygenated water in the first fluid circuit 406, which is downstream from the second heat exchanger 416. This method of temperature control causes the de-oxygenated water in the storage reservoir 401 to be maintained at a lower temperature of between 5° C. and 10° C. below the actual shut-off temperature setting of thermostat sensor 429.

As will be appreciated, at the completion of mains water heating mode, the cooler mains water, which is trapped in the second fluid circuit 410 after a tap (not shown) downstream of the water heater 400 is closed, will continue to heat and expand. As the trapped mains water expands, pressure in the second fluid circuit 410 is released by allowing small injections of mains water into the first fluid circuit 406 through expansion valve 422. These small injections of mains water also serve to top up and maintain the water level in the first fluid circuit 406 to account for any losses. If the water level in the first fluid circuit 406 becomes too large, it simply overflows via overflow 402 of the storage reservoir 401.

FIG. 35 shows a wiring diagram of the water heater 400 when in mains water heating heating mode, with sensor 429 sensing a de-oxygenated water temperature below 82° C. whilst sensor 430 is sensing mains water below 50° C. Accordingly sensor 430 connects the pump 423 to the active power line 510 which is independent and not influenced by sensor 429. The components shown in the wiring diagram are the gas fuel valve controller 500, mains power input 510, a 7.5 Amp fuse 512, pump 423, forced draft fan 441, burner thermostat sensor 429 a, burner thermostat switch 429 b, pump thermostat sensor 430 a, pump thermostat switch 430 b, safety cut-out sensor 432 a, safety cut-out switch 432 b, line inlet 602, active inlet 603, neutral inlet 604 a, neutral wire 604 b, earth inlet 605 a, earth wire 605 b, ignition wire 606 and combustion flames 607.

In maintenance heating modes, as shown in FIGS. 23 36 and 37, the water heater 400 is not being called upon to supply hot mains water. The de-oxygenated water is periodically cycled through the first 405, second 416 and third 409 heat exchangers. The burner 404 and pump 423 are periodically actuated and deactivated simultaneously in response to feedback from thermostat sensor 429 to maintain the de-oxygenated water in the storage reservoir 401 at a temperature of between 68° C. and 70° C.

FIG. 36 shows the wiring diagram of the water heater 400 when in maintenance heating mode and with thermostat sensor 429 sensing a de-oxygenated water temperature above its shut off setting. The water heater 400 is in a standby maintenance mode with no water being circulated through the unit and burner 404 and pump 423 off. The components shown in the wiring diagram are the gas fuel valve controller 500, mains power input 510, a 7.5 Amp fuse 512, pump 423, forced draft fan 441, burner thermostat sensor 429 a, burner thermostat switch 429 b, pump thermostat sensor 430 a, pump thermostat switch 430 b, safety cut-out sensor 432 a, safety cut-out switch 432 b, gas line inlet 602, active inlet 603, neutral inlet 604 a, neutral wire 604 b, earth inlet 605 a, earth wire 605 b and ignition wire 606.

FIG. 37 shows the wiring diagram of the water heater 400 when in maintenance heating mode and with thermostat sensor 429 sensing a de-oxygenated water temperature below its shut off setting. Pump 423 is receiving its active power from and active line 510 via the gas controller which also supplies power to the combustion fan 441 and both are activated and deactivated by controlling thermostat 429. The components shown in the wiring diagram are the gas fuel valve controller 500, mains power input 510, a 7.5 Amp fuse 512, pump 423, forced draft fan 441, burner thermostat sensor 429 a, burner thermostat switch 429 b, pump thermostat sensor 430 a, pump thermostat switch 430 b, safety cut-out sensor 432 a, safety cut-out switch 432 b, gas line inlet 602, active inlet 603, neutral inlet 604 a, neutral wire 604 b, earth inlet 605 a, earth wire 605 b, ignition wire 606 and combustion flames 607.

FIG. 24 shows a hydronic heating mode of the water heater 400. In response to the hydronic heating mode being actuated, the hydronic controller (not shown) simultaneously activates pumps 434, 435 and 423.

Pump 434 prevents de-oxygenated water from flowing from the storage reservoir 401 directly into the first heat exchanger 405, because the flow of water through the third fluid circuit 426 feeds into the first fluid circuit 406 between the tank reservoir 401 and the first heat exchanger 405 and is sufficient to supply the demand from pump 423. This ensures that the temperature differential between the combustion products in the first heat exchanger 405 and the de-oxygenated water (which has been cooled by the hydronic system in plate heat exchanger 427) is the largest achievable and consequently the highest possible combustion/thermal efficiency can be maintained during hydronic heating mode.

The de-oxygenated water is continuously cycled through the first heat exchanger 405, the second heat exchanger 416, into the storage reservoir 401, exiting via outlet 425, which directs the de-oxygenated water into pump 434, which pumps the deoxygenated water through the plate heat exchanger 427. The burner 404 is periodically actuated and deactivated by thermostat sensor 429 in response to the temperature of the de-oxygenated water circulating through the first 406 and third fluid circuit 426.

The hydronic controller (not shown) is also fitted with a mains water priority sensor (not shown), which automatically shuts down pumps 434 and 435 should the mains water outlet temperature ever drop below the user's desired preset hot water temperature. The cut-off setting for the hydronic heating system is also adjustable to suit the operating temperature of the hydronic system and ensures that the domestic mains water is always given priority over the hydronic heating system should the unit not be able to supply sufficient energy to run both functions.

The expansion valve 422 operates in the same manner in the hydronic heating mode as in the maintenance heating mode and does not inject any water into the first fluid circuit 406.

Ventilation of the water heater 400 will now be described with reference to FIG. 21.

During operation of the burner 404, in response to feedback from the thermostat sensor 429, the controller (not shown) actuates a forced draft fan 441 to draw combustion air into a space below the combustion chamber 403 and then through the fan 441, where the combustion air is mixed with burner fuel for ignition.

This flow path of the incoming air enables the exterior surfaces of the lower part of the combustion chamber 403 and other heat sensitive internal components, such as components of the fuel valve 433, fan 441, pump 423 and thermostat sensors 429 and 430, to be air cooled by the ambient air drawn in from outside the water heater 400.

Cooler air inside the bottom of the housing 437 is drawn through vent holes 444 via air inlet vent 442. The air vent holes 444 direct the incoming air directly onto components of the fuel valve 433, fan 441, pump 423 and thermostat sensors 429 and 430 whilst the burner 404 is in operation. The heated cooling air is then re-directed back into the combustion chamber 403, through the second 416 heat exchanger, and then through the first heat exchanger 405, via the forced draft fan 441.

High temperature combustion products 445, shown as vertical arrows, are forced up to make contact with the second heat exchanger 416 at the top of the combustion chamber 403. The high temperature combustion products 445 heat exchange with the de-oxygenated water in the heat exchanger 416, which is always flowing when the burner 404 is on.

It will be appreciated that the combustion products 446 that have passed through the second heat exchanger 416 have heat exchanged with the de-oxygenated water in the second heat exchanger 416, and have been significantly cooled. These cooler combustion products 446 then rise and pass through the first heat exchanger 405, where they heat exchange predominantly with the mains water in the outer thermally conductive pipes 414 b (as shown in FIG. 21).

As the combustion products 446 pass through the first heat exchanger 405, they enter flue outlet duct 443, which forces them vertically down to the flue outlet 447, which directs the cooled combustion products 448 horizontally out and away from a front cover 449 of the housing 437.

As the combustion products 446 cool in the first heat exchanger 405, they form condensation droplets on the surface of the vertically orientated heat exchanging fins 415. The condensation droplets flow vertically down the fins 415 of the first heat exchanger 405 and collect in a condensation bath 450 at the base of the first heat exchanger 405. The condensation then flows through a drain outlet (not shown) in the base of the condensation bath 450 and out of the water heater 400 via a drain pipe (not shown).

The condensation bath 450 is positioned so that it is not in contact with any hot surfaces or in the path of any high temperature combustion products which ensures that the condensate will not re-evaporate or form a vapour. The condensate liquid drains from the water heater 400 at a relatively low temperature.

Filling/priming of the water heater 400 will now be described with reference to FIGS. 25-27.

After the water heater 400 has been installed and all the water and gas pipes are connected, the installer opens the expansion valve 422 by pulling on a lever 451 at the top of the valve 422. This injects mains pressure water into the de-oxygenated first fluid circuit 406, as shown by the arrows, which demonstrate the filling/priming flow path of the mains water into the de-oxygenated first fluid circuit 406.

As can be seen in FIG. 25, the mains water injected from the expansion valve 422 flows into the de-oxygenated first fluid circuit 406 and takes two paths at junction 452 upon entering the de-oxygenated first fluid circuit 406. The outlet of expansion valve 422 is set at an angle so that the majority of the mains water entering and filling the de-oxygenated first fluid circuit 406 is directed at pump 423 to ensure that the first heat exchanger 405 can be purged of air that is driven out by the rising mains water as it makes its way through the de-oxygenated first fluid circuit 406, the first heat exchanger 405 and ultimately into the storage reservoir 401 via the third heat exchanger inlets 420 and bleed holes 408 in a reverse direction to the normal pump 423 flow direction.

As shown in FIG. 25, some of the mains water will flow in the direction of the combustion chamber 403 and initially will flow through to the storage reservoir 401 via the first fluid circuit return pipe 407. Initially, the majority of the mains water will flow through the pump 423 and into the storage reservoir 401 via the first heat exchanger 405 and the third heat exchanger 419 in a reverse direction to the normal pump 423 flow, as shown by the arrows which indicate the mains water flow.

FIG. 26 shows that, as the storage reservoir 401 starts to fill up with mains water that enters via the third heat exchanger 409 and the first fluid circuit return pipe 407, the outlet from the first fluid circuit return pipe 407 becomes submerged by the rising mains water in the storage reservoir 401, as indicated by the water level 453. Because the outlet of the first fluid circuit return pipe 407 is submerged, the pressure rises in the first fluid circuit return pipe 407 and continues to increase as the mains water level rises in the storage reservoir 401. This increase of pressure creates a resistance which prevents the residual air in the combustion chamber 403 and the first fluid circuit return pipe 407 from being purged or driven out during the priming process.

Consequently, all the mains water entering from the expansion valve 422 is now forced to enter the storage reservoir 401 via the first heat exchanger 405 and the third heat exchanger 409 in a reverse direction to the normal pump 423 flow, as shown by the arrows which indicate the mains water flow. This feature ensures that all the air is purged from the first heat exchanger 405 and the third heat exchanger 409, which is vital to ensure that, after the priming process is completed, pump 423 can draw (suck) the mains water from the storage reservoir 401 and circulate it through the first fluid circuit during normal operation. The bleed holes 408 also ensure that the air that is being purged from the first heat exchanger 405 and the third heat exchanger 419 can escape during the filling process despite the rising water level in the storage reservoir 401.

FIG. 27 shows the water heater 400 at the completion of the filling sequence. Mains water can be seen flowing out of the overflow outlet 402, which indicates that the storage reservoir is full of water and that first heat exchanger 405 and the third heat exchanger 409 are purged of air and full of mains water.

After the installer sees the mains water flowing out of the overflow outlet 402 he closes the expansion valve 422 by releasing the lever 451 at the top of the valve 422. After the installer has closed the expansion valve 422 air from outside the water heater 400 enters into the overflow outlet air vent 609 (as shown in FIG. 38) via overflow drain air vent outlet 612 which allows the water in the overflow drain pipe 613 to drain out of the water heater 400. Overflow outlet air vent 609 prevents any syphoning of water stored in the storage reservoir 401 by allowing air to enter the overflow drain pipe 613. Water traps 608 and 610 also prevent large vapour and heat losses from the water heater 400 during normal operational modes.

At this stage of the priming sequence, there is still air present in the combustion chamber 403 and the first fluid circuit return pipe 407.

At this point the installer connects the power to the water heater 400. The thermostat sensor 429 (shown in FIG. 22) will instantly sense that the water heater 400 is cold and cause the controller (not shown) to activate the fuel valve 433, which will initiate an ignition sequence which causes pump 423 and fan 441 to start simultaneously. The pump 423 will draw water from the storage reservoir 401 through the first fluid circuit 406 via the third heat exchanger 419 and first heat exchanger 405. As the pump 423 activates, all the residual air which was previously trapped in the combustion chamber 403 and the first fluid circuit return pipe 407 is driven out by the water being pumped into the storage reservoir 401. As the water is heated in the first fluid circuit the oxygen present in the water is driven out and ultimately becomes de-oxygenated.

The above described embodiment provides the following benefits:

-   -   The pipe-in-pipe configuration allows for both the mains water         and de-oxygenated water to be utilised as a conduit medium for         heat transfer, which provides a high efficiency performance in         mains water heating, maintenance and hydronic heating modes.     -   The pipe-in-pipe configuration enables the water heater to         suffer virtually no negative effects in terms of longevity if         the water supply is corrupted with impurities, as the mains         water circuit 410, which potentially contains impurities, is         separated from the de-oxygenated water circuit 406 and         consequently the potential for scaling is significantly reduced,         because the mains water circuit 410 does not into contact with         the high temperature combustion products 445, as shown in FIG.         22.     -   The de-oxygenated water in the water heater 400 is not replaced         and is constantly re-circulated between the storage reservoir         401 and the heat exchangers 405, 416, 409. Accordingly, minimal         topping up of the de-oxygenated water is required, which thereby         reduces the probability of the de-oxygenated water being         contaminated.     -   In mains water heating mode, the large temperature differential         between the flowing cold mains water and the combustion products         446 in the first heat exchanger 405 provides a         combustion/thermal efficiency of approximately 94-98%, dependent         on the mains water inlet temperature.     -   In maintenance mode, the temperature differential between the         recirculating de-oxygenated water and the combustion products         446 is not as large as during mains water heating mode. However,         there is still a large increase in efficiency contributed via         the de-oxygenated water flowing through the first heat exchanger         405. Accordingly, the combustion/thermal efficiency in         maintenance mode is between 90% and 94%, dependent on the         temperature setting of de-oxygenated water in the storage         reservoir 401.     -   In hydronic heating mode, by isolating and delivering the         returned (cooled) de-oxygenated water from the return line 454         from the fourth heat exchanger 427 directly into the first heat         exchanger 405, a large temperature differential is created with         the combustion products 446. This large temperature differential         can again produce a high efficiency performance. However, the         operating temperature of the heating system has a direct bearing         on the efficiency of the water heater 400 in hydronic heating         mode. If the hydronic heating system temperature is set above         80° C., the water heater will provide a thermal/combustion         efficiency close to 90%. If the hydronic heating system is set         under 60-50° C., or the returning water from the hydronic         heating system is lower than 45-50° C., the water heater 400 can         provide high efficiency combustion/thermal performance         (condensing) of between 90 and 95%.     -   The ability of the water heater 400 to continually cool the         first heat exchanger 405, in circumstances whereby the burner         404 is cycled on and off, by continuously cycling cold mains         water through the first heat exchanger 405.     -   If the water heater 400 is functioning as a domestic water         heater and the flow rate (energy requirement) is lower than the         water heater's capacity, the burner 404 will cycle on and off to         ensure an appropriate/desired hot mains water temperature can be         supplied.     -   The air flow path inside the water heater 400 ensures that heat         losses from the combustion chamber 403 are re-directed back into         the combustion chamber 403, where the heat is recovered via the         first 405 and second 416 heat exchangers.     -   The inlet and outlet ports from the storage reservoir 401 all         penetrate via the reservoir's horizontal top lid, so as not to         contact de-oxygenated water within the reservoir 401. By         positioning the exit and entry points of the inlet and outlets         in the air gap 455, as shown in FIG. 22, at the top of the         reservoir 401, the potential for leaks to occur is reduced. This         design also enables a variety of alternate materials to be used         for the reservoir 401.     -   The use and function of dual thermostat sensors 429 and 430 also         prevents nuisance burns (stair casing), which can occur in         single thermostat systems.     -   As thermostat sensor 430 senses mains water flow by a thermal         reading and not a flow reading, it can operate at any mains         domestic water flow rate or pressure.     -   When the burner 404 is off, the heater 400 is able to maintain a         consistent hot mains water output due to the heat energy stored         in the relatively hot de-oxygenated water stored in the         reservoir 401, which is pumped through the various heat         exchangers 405, 416, 409 to heat the mains water until the         burner 404 has activated.     -   The mixing of de-oxygenated water returning to the reservoir 401         with de-oxygenated water already in the reservoir 401 is         enhanced by the internal currents inside reservoir 401, which         are caused by the velocity of the circulation pump 423 and the         direction of the returning de-oxygenated water, which is set by         the return pipe 407. This mixing ensures that the reservoir 401         supplies de-oxygenated water of a consistent temperature, which         reduces temperature fluctuations in the hot flowing mains water         whilst burner 404 is cycling on and off in mains water heating         mode.

It will be appreciated that numerous modifications can be made to the second embodiment. For example:

-   -   in the first heat exchanger 405, the first thermally conductive         pipe 414 a can be adapted to carry the mains water and the         second thermally conductive pipe 414 b can be adapted to carry         the de-oxygenated water;     -   in the third heat exchanger 409, the thermally conductive pipe         409 a can be adapted to carry the de-oxygenated water and the         thermally conductive pipe 409 b can be adapted to carry the         mains water;     -   the gas burner 404 can be replaced with an electric burner, an         oil burner or another type of fuel burner;     -   the fourth heat exchanger 427 can be a radiator panel, air         convector (forced draft or natural convection), or         underfloor/slab heating coils;     -   the water heater 400 preferably only has one gas input rate,         which is the maximum rate as determined by the burner fuel valve         433 settings. However, alternative gas or oil management systems         (modulating input rates) can be applied to the water heater 400;     -   the atmospheric pressure storage reservoir 401 can be replaced         with a pressurised storage reservoir governed by a conventional         pressure and temperature relief/safety valve; and/or     -   the upper and lower limits of the various sensors can be         adjusted based on the user's requirements and/or operating         parameters of the water heater (eg. desired hot water output         temperature, cold mains water input temperature). 

1. A fluid heater comprising: a storage reservoir for storing heating fluid; a burner for heating the heating fluid; a first heat exchanger located outside said storage reservoir; a first fluid circuit connecting the storage reservoir with the first heat exchanger for carrying heating fluid from the storage reservoir, through the first heat exchanger and back to the storage reservoir; and a second fluid circuit connecting a first inlet for mains fluid to a first outlet for heated mains fluid via the first heat exchanger, wherein the first heat exchanger is adapted to exchange heat between said heating fluid and said mains fluid.
 2. A fluid heater according to claim 1, wherein the first heat exchanger includes a first pipe-in-pipe heat exchanger.
 3. A fluid heater according to claim 2, wherein the first pipe-in-pipe heat exchanger includes a first thermally conductive pipe for containing the heating fluid, said first thermally conductive pipe being located within a second thermally conductive pipe for containing the mains fluid.
 4. A fluid heater according to claim 2, wherein the first pipe-in-pipe heat exchanger includes a first thermally conductive pipe for containing the mains fluid, said first thermally conductive pipe being located within a second thermally conductive pipe for containing the heating fluid.
 5. A fluid heater according to claim 3, wherein the first heat exchanger includes a plurality of the first and second thermally conductive pipes, with each said first thermally conductive pipe being located within a respective said second thermally conductive pipe.
 6. A fluid heater according to claim 5, wherein the first thermally conductive pipes are connected in parallel and the second thermally conductive pipes loop backward and forward through the first heat exchanger.
 7. A fluid heater according to claim 1, wherein the first heat exchanger is adapted to be heated by combustion products generated by the burner.
 8. A fluid heater according to claim 7, wherein the burner is a gas burner and the first heat exchanger is located above the gas burner, such that combustion products from the gas burner rise toward the first heat exchanger.
 9. A fluid heater according to claim 1, further comprising a second heat exchanger, downstream of the first heat exchanger with respect to a flow direction of the heating fluid, for exchanging heat between the heating fluid and the mains fluid.
 10. A fluid heater according to claim 9, wherein the second heat exchanger is located below the first heat exchanger, such that combustion products passing the second heat exchanger rise toward the first heat exchanger to heat the first heat exchanger.
 11. A fluid heater according to claim 9, wherein the second heat exchanger includes a second pipe-in-pipe heat exchanger.
 12. A fluid heater according to claim 11, wherein the second pipe-in-pipe heat exchanger includes a third thermally conductive pipe for containing the mains fluid, said third thermally conductive pipe being located within a fourth thermally conductive pipe for containing the heating fluid.
 13. A fluid heater according to claim 11, wherein the second pipe-in-pipe heat exchanger includes a third thermally conductive pipe for containing the heating fluid, said third thermally conductive pipe being located within a fourth thermally conductive pipe for containing the mains fluid.
 14. A fluid heater according to claim 9, further comprising a third heat exchanger, downstream of the first heat exchanger with respect to a flow direction of the heating fluid, for exchanging heat between the heating fluid and the mains fluid.
 15. A fluid heater according to claim 14, wherein said third heat exchanger is downstream of the second heat exchanger with respect to a flow direction of the heating fluid.
 16. A fluid heater according to claim 14, wherein the third heat exchanger is located above the second heat exchanger.
 17. A fluid heater according to claim 14, wherein the third heat exchanger is located in the storage reservoir.
 18. A fluid heater according to claim 1, further comprising a first valve for selectively preventing flow of the heating fluid through the first heat exchanger.
 19. A fluid heater according to claim 18, further comprising a second valve for selectively preventing flow of the heating fluid from the storage reservoir.
 20. A fluid heater according to claim 1, further comprising an expansion valve between the second fluid circuit and the first fluid circuit to allow mains fluid to pass from the second fluid circuit into the first fluid circuit when pressure within the second fluid circuit increases to a predetermined level.
 21. A fluid heater according to claim 18, further comprising a controller adapted to close the first valve, in response to a first flow sensor sensing flow of mains fluid, to thereby prevent heating fluid from flowing through the first heat exchanger.
 22. A fluid heater according to claim 21, wherein the controller is adapted to actuate a heating sequence of the burner in response to the first flow sensor sensing flow of mains fluid and a first temperature sensor indicating a heating fluid temperature below a first predetermined value.
 23. A fluid heater according to claim 22, wherein the first predetermined value is approximately 82° C.
 24. A fluid heater according to claim 21, wherein the controller is adapted to deactivate the burner in response to the first flow sensor sensing flow of mains fluid and a first temperature sensor indicating a heating fluid temperature above a second predetermined value.
 25. A fluid heater according to claim 24, wherein the second predetermined temperature is approximately 85° C.
 26. A fluid heater according to claim 21, wherein the controller is adapted to open the first valve in response to the first flow sensor sensing a stoppage of mains fluid flow, to thereby allow heating fluid to flow through the first heat exchanger to scavenge residual heat in the first heat exchanger.
 27. A fluid heater according to claim 21, wherein the controller is adapted to actuate the burner in response to the first flow sensor sensing a stoppage of mains fluid flow and a second temperature sensor indicating a heating fluid temperature below a third predetermined value.
 28. A fluid heater according to claim 27, wherein the third predetermined value is approximately 65° C.
 29. A fluid heater according to claim 21, wherein the controller is adapted to deactivate the burner in response to the first flow sensor sensing a stoppage of mains fluid flow and a second temperature sensor indicating a heating fluid temperature above a fourth predetermined value.
 30. A fluid heater according to claim 29, wherein the fourth predetermined temperature is approximately 80° C.
 31. A fluid heater comprising: a storage reservoir for storing heating fluid; a burner for heating the heating fluid; a first heat exchanger; a first fluid circuit connecting the storage reservoir with the first heat exchanger for carrying heating fluid from the storage reservoir, through the first heat exchanger and back to the storage reservoir; and a second fluid circuit connecting an inlet for mains fluid to an outlet for heated mains fluid via the first heat exchanger, the second fluid circuit including a portion that passes through an exhaust path of burner heat expelled after heating of the heating fluid, wherein the first heat exchanger is adapted to exchange heat between said heating fluid and said mains fluid.
 32. A fluid heater according to claim 31, wherein the first heat exchanger includes a first pipe-in-pipe heat exchanger.
 33. A fluid heater according to claim 32, wherein the first pipe-in-pipe heat exchanger includes a first thermally conductive pipe for containing the heating fluid, said first thermally conductive pipe being located within a second thermally conductive pipe for containing the mains fluid.
 34. A fluid heater according to claim 32, wherein the first pipe-in-pipe heat exchanger includes a first thermally conductive pipe for containing the mains fluid, said first thermally conductive pipe being located within a second thermally conductive pipe for containing the heating fluid.
 35. A fluid heater according to claim 33, wherein the first heat exchanger includes a plurality of the first and second thermally conductive pipes, with each said first thermally conductive pipe being located within a respective said second thermally conductive pipe.
 36. A fluid heater according to claim 35, wherein the first thermally conductive pipes are connected in parallel and the second thermally conductive pipes loop backward and forward through the first heat exchanger.
 37. A fluid heater according to claim 31, wherein the first heat exchanger is adapted to be heated by combustion products generated by the burner.
 38. A fluid heater according to claim 37, wherein the burner is a gas burner and the first heat exchanger is located above the gas burner, such that combustion products from the gas burner rise toward the first heat exchanger.
 39. A fluid heater according to claim 31, further comprising a second heat exchanger, downstream of the first heat exchanger with respect to a flow direction of the heating fluid, for exchanging heat between the heating fluid and the mains fluid
 40. A fluid heater according to claim 39, wherein the second heat exchanger is located below the first heat exchanger, such that combustion products passing the second heat exchanger rise toward the first heat exchanger to heat the first heat exchanger.
 41. A fluid heater according to claim 39, wherein the second heat exchanger includes a second pipe-in-pipe heat exchanger.
 42. A fluid heater according to claim 41, wherein the second pipe-in-pipe heat exchanger includes a third thermally conductive pipe for containing the mains fluid, said third thermally conductive pipe being located within a fourth thermally conductive pipe for containing the heating fluid.
 43. A fluid heater according to claim 41, wherein the second pipe-in-pipe heat exchanger includes a third thermally conductive pipe for containing the heating fluid, said third thermally conductive pipe being located within a fourth thermally conductive pipe for containing the mains fluid.
 44. A fluid heater according to claim 39, further comprising a third heat exchanger, downstream of the first heat exchanger with respect to a flow direction of the heating fluid, for exchanging heat between the heating fluid and the mains fluid.
 45. A fluid heater according to claim 44, wherein said third heat exchanger is downstream of the second heat exchanger with respect to a flow direction of the heating fluid.
 46. A fluid heater according to claim 44, wherein the third heat exchanger is located above the second heat exchanger.
 47. A fluid heater according to claim 44, wherein the third heat exchanger is located in the storage reservoir.
 48. A fluid heater according to claim 31, further comprising a first valve for selectively preventing flow of the heating fluid through the first heat exchanger.
 49. A fluid heater according to claim 31, further comprising a second valve for selectively preventing flow of the heating fluid from the storage reservoir.
 50. A fluid heater according to claim 31, further comprising an expansion valve between the second fluid circuit and the first fluid circuit to allow mains fluid to pass from the second fluid circuit into the first fluid circuit when pressure within the second fluid circuit increases to a predetermined level.
 51. A fluid heater according to claim 48, further comprising a controller adapted to close the first valve, in response to a first flow sensor sensing flow of mains fluid, to thereby prevent heating fluid from flowing through the first heat exchanger.
 52. A fluid heater according to claim 51, wherein the controller is adapted to actuate a heating sequence of the burner in response to the first flow sensor sensing flow of mains fluid and a first temperature sensor indicating a heating fluid temperature below a first predetermined value.
 53. A fluid heater according to claim 52, wherein the first predetermined value is approximately 82° C.
 54. A fluid heater according to claim 51, wherein the controller is adapted to deactivate the burner in response to the first flow sensor sensing flow of mains fluid and a first temperature sensor indicating a heating fluid temperature above a second predetermined value.
 55. A fluid heater according to claim 54, wherein the second predetermined temperature is approximately 85° C.
 56. A fluid heater according to claim 51, wherein the controller is adapted to open the first valve in response to the first flow sensor sensing a stoppage of mains fluid flow, to thereby allow heating fluid to flow through the first heat exchanger to scavenge residual heat in the first heat exchanger.
 57. A fluid heater according to claim 51, wherein the controller is adapted to actuate the burner in response to the first flow sensor sensing a stoppage of mains fluid flow and a second temperature sensor indicating a heating fluid temperature below a third predetermined value.
 58. A fluid heater according to claim 57, wherein the third predetermined value is approximately 65° C.
 59. A fluid heater according to claim 51, wherein the controller is adapted to deactivate the burner in response to the first flow sensor sensing a stoppage of mains fluid flow and a second temperature sensor indicating a heating fluid temperature above a fourth predetermined value.
 60. A fluid heater according to claim 59, wherein the fourth predetermined temperature is approximately 80° C.
 61. A fluid heater according to claim 31, wherein the first heat exchanger is located outside the storage reservoir.
 62. A method for heating fluid, said method comprising the steps of: providing a supply of heating fluid; heating the heating fluid; providing mains fluid; heating the mains fluid by exchanging heat between said heating fluid and said mains fluid; and further heating the mains fluid using residual heat from the step of heating the heating fluid.
 63. A method according to claim 62, including the further step of reheating the heating fluid if the temperature of the heating fluid falls below a predetermined lower limit.
 64. A method according to claim 62, including the further step of stopping the heating step if the temperature of the heating fluid reaches a predetermined upper limit.
 65. A method according to claim 63, including the step of changing the lower limit based on whether mains fluid is flowing.
 66. A method according to claim 64, including the step of changing the upper limit based on whether mains fluid is flowing.
 67. A method according to claim 62, including the additional step of heating fluid in an external hydronic heating circuit by exchanging heat between the heating fluid in the first fluid circuit and fluid in the hydronic heating circuit.
 68. A method according to claim 67, including the additional step of stopping the exchange of heat between the heating fluid in the first fluid circuit and fluid in the hydronic heating circuit if a temperature of the heating fluid in the first fluid circuit falls below a predetermined lower limit. 